In collaboration with investigators from NIMH, a fully quantitative method for determining regional rates of cerebral protein synthesis with positron emission tomography (PET) was previously developed. This method was adapted from the autoradiographic L-1-C-14leucine method. It uses L-1-C-11leucine as the PET radiotracer, dynamic PET scanning, and a kinetic model to measure cerebral protein synthesis rate. Studies were performed in nonhuman primates with a High Resolution Research Tomograph (HRRT), which acquires brain images with 2.5 mm resolution. Paired scans performed at baseline and with increased plasma phenylalanine concentration were analyzed to determine the effect of large neutral amino acid concentration on the estimated model parameters. Measured transport constants from blood to brain changed, consistent with competitive inhibition of transport of leucine into brain by phenylalanine. Measured cerebral protein synthesis rate remained constant however. These results were consistent with invasive studies reported in other species, and provided additional validation of the PET method, which is currently being used in human subjects.